The present invention generally relates to NMOS drivers, and more specifically relates to a self-ballasting Electro Static Discharge (ESD) protection technique in a fully silicided CMOS process.
In CMOS technologies, non-silicided grounded-gate NMOS fingers (ggNMOSFET's) are widely used as the primary ESD device in I/O buffers and power rail clamps because of the high ballasting resistance effect. By designing drain-contact-to-gate spacing (DCGS), non-silicided NMOS fingers can provide very high ESD failure threshold.
In contrast, in fully silicided CMOS process (such as in LSI Logic's G10 technology), silicide block mask is not available, so an ESD robust design cannot be achieved because silicided NMOS fingers have very weak ESD failure threshold due to non-uniform trigger of parasitic N-P-N bipolar transistors. Although P/N junction diodes or silicon controlled rectifiers (SCR) can be used as the primary ESD device in output buffers, the ESD power clamp still becomes an issue. ESD weakness of silicided ggNMOSFET's is mainly due to non-uniform silicidation process and low ballasting resistance.
FIG. 1 provides a top view of conventional silicided NMOS fingers 10. As shown, conventional NMOS fingers 10 generally includes a source 12, gates (or poly gates) 14, 16 and drains 18, 20. The source 12 includes source contacts 22, and the drains 18, 20 include drain contacts 24, 26, respectively. Proximate each of the drains 18, 20, generally between the drain contacts 24, 26 and the gates 14, 16 is non-segmented drain diffusion 30, 32. In other words, non-segmented drain diffusion 30 extends generally outwardly from drain 18 toward gate 14, and non-segmented drain diffusion 32 extends outwardly from drain 20 toward gate 16. Similarly, proximate the source 12, generally between the source contacts 22 and the gates 14, 16, is source diffusion 34.
As discussed above, conventional silicided ggNMOSFET's, such as is shown in FIG. 1, have weak ESD protection, mainly due to non-uniform silicidation process and low ballasting resistance.